1. Field of the Invention
The present invention relates generally to the field of photoablation of ocular tissue to correct vision deficiencies and treat other vision-impairing ocular problems and, more particularly, to treatment of the natural ocular lens.
2. Background Discussion
Historically, and until only a few decades ago, eyeglasses (i.e., spectacles) were exclusively used for most correctable vision deficiencies, including, for example, hyperopia (wherein incident parallel rays of light converge to focus behind the retina), myopia (wherein incident parallel rays of light converge to a focus in front of the retina), and astigmatism (a defect in vision ordinarily caused by irregularities in the cornea). However, in about the 1940s, contact lens started being used as a viable alternative, at least for many individuals, to the use of spectacles for correcting vision deficiencies, and provided--often at a cost of some discomfort--freedom from many annoyances and appearance problems associated with the wearing of spectacles.
Another method for treating some types of problems causing vision problems was introduced by Dr. Peter Ridley just after the close of World War II. This new (although there is some evidence that it had been tried several hundred years ago) method involved the replacement of a diseased natural ocular lens, for example, a natural lens which had been clouded because of cataract, with a plastic artificial or prosthetic intraocular lens (IOL). Such lens extraction and IOL implantation is now a commonly-performed surgical procedure and is credited with saving the sight of many individuals who were or would have become blind.
Vision correction can now be achieved on some patients, especially those with myopia, by a surgical procedure on the cornea called radial keratotomy (RK). In an RK procedure, several slits (for example, about four to about eight) are made radially inwardly toward the optical axis from the peripheral edge of the cornea. These radial slits enable the cornea to flatten out a bit, thereby decreasing the curvature of the cornea. Candidates for RK procedures are typically nearsighted individuals who cannot or who do not want to wear either spectacles or contact lenses.
Corneal onlays or implants, which may be constructed of synthetic materials or from donor corneas, which are surgically attached to or implanted into patients'eyes, are also useful to enhance vision in patients whose corneas have been damaged and/or scarred by corneal diseases, such as ulcers or cancer, or by injury to the cornea.
Because of the shortcomings associated with RK surgery and a desire to provide vision correction to many individuals without the necessity for those individuals to wear spectacles or contact lenses, considerable research and development have been directed over the past several years to apparatus and techniques for reshaping the anterior (forward) surface of the cornea.
Excimer lasers--lasers operating in the ultraviolet (UV) region of less than about 200 nanometers wavelength--have thus now been used to selectively ablate regions of the cornea to resculpture the corneas of patients in a manner correcting certain vision problems. For example, regions of the cornea around its optical axis are photoablated to a greater depth than peripheral regions of the cornea, thereby decreasing the curvature of the cornea to correct myopia.
In contrast, photoablation of the cornea is concentrated near the periphery of the cornea to increase the curvature of the cornea and thereby correct for hyperopia. In a related manner, astigmatism can be corrected by selectively varying the rate of laser photoablation of an astigmatic cornea in a manner providing an appropriate vision correction. In this regard, U.S. Pat. No. 4,784,135 to Blum et al. discloses a method for removing biological tissue by irradiation of the tissue with UV radiation while, for example, U.S. Pat. Nos. 4,665,913; 4,669,466; 4,718,418; 4,721,379; 4,729,372; 4,732,149; 4,770,172; 4,773,414; and 4,798,204 to L' Esperance disclose apparatus and methods for laser sculpting of corneal tissue to correct vision defects.
In addition, U.S. Pat. No. 4,842,782 to Portney et al. and U.S. Pat. No. 4,856,513 to Muller (as well as one or more of the above-cited L' Esperance patents) disclose masks useful for selectively controlling the laser beam intensity or total laser beam energy to different regions to thereby enable selective corneal ablation to effect the desired vision correction. Various of the above-cited patents to L' Esperance also disclose methods for determining the required laser ablation profile for the cornea. For example, U.S. Pat. No. 4,995,923 discloses computer mapping of the cornea and computer-controlled scanning of the cornea by the laser beam.
In spite of reported short-term medical successes--both in clinical testing in the United States and in use in unregulated foreign countries--with laser photoablation of corneal tissue to correct vision deficiencies, the verdict is still not in concerning the long-term effects and efficacy of corneal laser photoablation.
In particular, questions have been raised whether over a long term the vision correction initially provided by photoablation of the cornea will remain effective because of the normal regrowth of the removed epithelium layer of the cornea over the ablated area. In this regard, there seems to be at least some natural tendency for the epithelium layer to regrow in a manner that, in time, the pre-ablation contour of the cornea may be reestablished sufficiently so that vision recorrection is required. An ancillary question is, therefore, how many times and how frequently can a laser photoablation process be repeated?
Also, there have been reports of haze forming on the cornea after photoablation. Although this appears to be a relatively transient phenomenon--lasting only a few months and ordinarily not too bothersome to the patient--at the present there has been insufficient post-ablation time on any patients to determine long term effects.
Moreover, it appears that there may be a maximum diopter change--around five diopters--that can presently be effectively and predictably made by corneal photoablation. Still further, at least at present, the laser ablation of corneal tissue is extremely painful to the patient on which the surgical procedure is performed.
Further, with respect to laser photoablation of the cornea, it should be appreciated that although in so doing, the cornea is sculpted in a manner correcting vision, it is frequently the case that the cornea itself is not responsible for the vision problems being corrected. As an illustration, myopia may more likely be caused by an increase in lens size, usually as a natural effect of the human aging process, of the natural lens of the eye (located posteriorly of the cornea). Other vision defects or deficiencies may also originate at the natural lens, while the associated cornea may itself be in a normal condition.
For these and other reasons, and for the reason that because the lens is closer to the retina than is the cornea, less material would have to be removed from the lens to achieve a similar vision correction, the present inventor has determined that it would often be preferable to reprofile the natural lens over reprofiling the cornea. Such natural lens reprofiling would eliminate many of the concerns presently raised about corneal photoablation and may result in reduced risks to patients; and since the lens has no nerve supply, the procedure should result in no sensation of pain to the patient.
It is therefore, a principal objective of the present invention to provide a method for laser ablation of selected regions of the natural lens in order to correct vision problems and to correct problems, such as incipient cataract, on the lens.